In the prior art, there has been an ever increasing demand, particularly by automobile manufacturers, for higher strength steel sheet and strip to provide both dent resistance and weight reduction in new automobile vehicle designs. With this desire, an increasing demand is seen for steels which are highly formable but also exhibit bake hardenability. As is well known in the art, bake hardenability refers to the strengthening that occurs in certain steels during the automotive paint baking treatment, typically around 350.degree. F. for 20 or 30 minutes. During the paint baking or other suitable treatment, a bake hardenable steel is strengthened to provide the desired dent resistance in the final product.
The attributes of ductility and strength are at conflict in a given steel. To achieve good formability (such as press formability or press shapability), the steel must be ductile in nature to be formed into the desired shape. Along with this ductility, however, the steel must also retain sufficient strength to resist denting when used in exposed panels such as those found in automobiles.
The prior art has proposed various solutions to overcome this conflict through the control of the steel alloying components as well as the process used for manufacturing the steel product. Bake hardenability is an attractive attribute contributing to these solutions because such hardening occurs after forming.
U.S. Pat. No. 5,133,815 to Hashimoto et al. discloses a cold-rolled or hot-dipped galvanized steel sheet for deep drawing. Bake hardenability is improved by control of the alloying steel components and a carburization step to obtain the proper concentration of solute carbon in the steel sheet.
U.S. Pat. No. 4,391,653 to Takechi et al. discloses a high strength cold-rolled strip having improved bake hardenability as a result of controlling the nitrogen content of the cold-rolled strip.
U.S. Pat. No. 4,496,400 to Irie et al. relates to cold-rolled steel sheets suitable for external automotive sheet. This patent discloses an effective compounding amount of niobium, which acts to fix C and N in the steel in the presence of a proper amount of aluminum and an annealing condition capable of developing effectively the contribution of niobium. Continuous annealing of this steel requires a detailed heating and cooling regimen to obtain the bake hardening effect.
U.S. Pat. No. 4,750,952 to Sato et al. also discloses a cold-rolled steel sheet having improved bake hardenability. In this patent, the amount of sulfur and nitrogen is limited and the addition of titanium is restricted to a specific range in consideration of the sulfur and nitrogen amounts. This patent also requires "time/energy intensive" annealing (i.e. greater than 300 seconds above recrystallization temperatures).
For automotive skin panel applications, coated steels such as hot dipped steels are preferred for their corrosion resistance. However, alloys especially suited for hot-dipped coating often have compositions which render them generally interstitial-free (IF). In these types of alloys, the alloying components effectively remove all of the carbon from solution which precludes bake hardenability.
Thus, a need has developed to provide improved methods and alloy chemistries which permit the manufacture of hot-dipped coated products which have both acceptable formability and bake hardenability properties. Further, in view of the need for precise chemistry controls with steel compositions utilizing alloying components such as titanium and/or niobium, a need has developed to provide an alloy chemistry suitable for bake hardening which does not require precise and extremely low alloy component limits and energy intensive processing requirements.
Responsive to this need, the present invention provides an improved hot-rolled or cold-rolled and annealed low carbon steel product suitable for sheet applications such as automotive sheet which has an alloy chemistry which is more easily controlled than prior art chemistries and also has less energy intensive and less demanding processing requirements.